Rather than stop for lunch, I'd recommend a picnic lunch that you can enjoy deep in the rainforest after a good hike. Fortunately, on the way to El Yunque along Route 3, you'll find one of Puerto Rico's better panaderías, or bakeries (but really, they're much more than just bakeries). Panadería Don Nico serves up a mean menu of sandwiches and local pastries...just what you need for your trip.
After the rainforest, continue along Route 3, heading east, and look out for signs for Luquillo Beach. A pleasant, beautifully kept and fully serviced public beach, Luquillo is wildly popular with Puerto Ricans, and it will make for a complete change from the rainforest.
For dinner, drive back along Route 3 until you find a string of roadside eateries. There are the famous kiosks of Luquillo, home to dozens of small shacks and tiny restaurants that serve up a blend of local specialties, finger foods, greasy snacks, and cheap drinks. It's a complete departure from yesterday's fine restaurant. The kiosks are rustic Puerto Rico at its best.
After dinner, most people will want to head home. The truly intrepid, however, might want to head east, to Fajardo. Call ahead at Yokahú Kayak Trips (787-604-7375), who will take you out into Fajardo's biobay, where you can take a glow-in-the-dark nighttime swim. It's an eerie but fantastic experience, if you're up for a late night out.
Fortunately, your last full day in Puerto Rico will be somewhat flexible.